This invention relates to a method for controlling and reducing the reaction zone in a titanium matrix composite.
Titanium matrix composites (TMC) are a class of high specific strength/stiffness, intermediate temperature composite structures developed for next generation aircraft turbine engines. The excellent properties of TMCs arise from the combination of high strength titanium alloy matrices with high strength/stiffness fibers. TMC reinforcements are typically monofilament silicon carbide fibers, such as SCS-6 from Textron Specialty Materials. These fibers have strengths in excess of 4 GPa, with Young's moduli over 400 GPa. Although these fibers have exceptional mechanical properties, they are extremely sensitive to flaws and cracks. Therefore, it is imperative that the fibers be protected as much as possible.
Protection of these monofilament fibers is afforded by a several-micron thick pyrolytic carbon-rich coating, which shields the fibers from damage in three ways: (1) as a sizing, it protects the fiber from handling damage; (2) as a weakly bonded mechanical interface, it deflects matrix cracks away from the fiber; and (3) as a sacrificial layer, it protects the silicon carbide portion of the fiber from reaction, and thus degradation, with the matrix.
Titanium matrix composites are typically fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. Under superplastic forming (SPF) conditions, which involve the simultaneous application of pressure and elevated temperature for a period of time, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus-contacting layer of matrix material bond together by a phenomenon known as diffusion bonding (DB).
Titanium matrix composites can also be fabricated by superplastic forming/diffusion bonding of a plurality of reinforcing fibers coated with the matrix material by vapor deposition or by sputtering. The matrix-coated fibers are consolidated by SPF/DB, as above.
Titanium matrix composites have not reached their full potential, at least in part, because of problems associated with instabilities at the fiber-matrix interface. At the time of high temperature bonding, a reaction can occur at the fiber-matrix interfaces, giving rise to what is generally called a reaction zone. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding. The reaction zone surrounding a filament introduces sites for easy crack initiation and propagation within the composite, which can operate in addition to existing sites introduced by the original distribution of defects in the filaments. It is well established that mechanical properties of metal matrix composites (MMC) are influenced by the reaction zone, and that, in general, these properties are degraded in proportion to the thickness of the reaction zone.
Control of the fiber/matrix reaction is a difficult, yet important problem to resolve, because of a basic conflict between the need to protect the fiber from excessive reaction versus optimization of composite consolidation parameters and matrix microstructural evolution. In order to ensure integrity of the carbon coating, TMCs utilizing SCS-6 are typically consolidated below about 1000.degree. C. However, optimization of the matrix microstructure requires solutionizing in the beta-phase field, which, for Ti-6Al-4V, is about 1066.degree. C.
One approach to protecting the fiber while enlarging the processing window consists of coating the fibers with diffusion barriers which slow the kinetics of the fiber/matrix reaction. While affording some protection to the fiber, these coatings add cost and mechanical complexity to the composite.
Another approach is to pre-process the matrix metal. Smith et al, U.S. Pat. No. 4,499,156, issued Feb. 12, 1985, disclose titanium alloy composites having a reduced reaction zone between the alloy matrix and the filament which is fabricated using a fine grain alloy sheet. Eylon et al, U.S. Pat. No. 4,733,816, issued Mar. 29, 1988, and Froes et al, U.S. Pat. No. 4,746,374, issued May 24, 1988, each disclose the use of rapidly solidified titanium alloy foil to produce composites having a reduced reaction zone.
Thus, a variety of choices are available to metal matrix composite fabricators. Together with these choices are several possible problems. On the one hand, if consolidation temperature is too low or consolidation pressure is too low, the resulting composite lacks desired strength due to incomplete bonding. On the other hand, if consolidation temperature is too high, the resulting composite may lack desired strength because of the resulting brittle reaction zone. If the thickness of the reaction zone is controlled, as disclosed by one of the aforementioned methods, the composite may lack certain desired physical properties.
Accordingly, what is desired is a method for controlling the interface between a titanium alloy matrix and a carbon-coated silicon carbide reinforcing filament or fiber.
It is an object of the present invention to provide a method for controlling the interface in a titanium matrix composite between the titanium alloy matrix and the carbon-coated silicon carbide reinforcing filament or fiber.
Other objects and advantages of the present invention will be apparent to those skilled in the art.